Researchers measured cortisol levels in the saliva of male undergraduate students as an indicator of HPA axis activity. They discovered that ‘dominance’ smiles, which challenge social standing and signal disapproval, were associated with higher HPA axis activity, such as increases in heart rate and salivary cortisol. Individuals perceiving ‘dominance’ smiles also took longer to return to their baseline cortisol levels after the stressful event. These physical responses to ‘dominance’ smiles mirror the influences of negative verbal feedback.